There are many designs of conventional radar altimeters for use in an aircraft. Many conventional radar altimeters are designed in multiple pieces, with most of the electronic components in a line replaceable unit (LRU) that connects via one or more cables to one or more antennas in separate units. Such antennas are mounted to an aircraft's skin and the LRU may also be mounted to the aircraft's skin or may be mounted somewhere else internally. Such radar altimeters require a service person to enter the inside of the aircraft for service and installation, and include multiple pieces to mount and maintain.
Other conventional radar altimeters have the electronics and antenna(s) integrated into a single package. Many of these radar altimeters are designed to mount to the internal surface of the aircraft's skin, and therefore still require a service person to enter inside the aircraft for service and installation. Also, many of these radar altimeters that integrate the electronics and antenna(s) have a large footprint on the aircraft's skin, which results in having to cut structure members for the aircraft's skin. Cutting the structural members reduces the structural integrity of those structural members, requiring a structure plate to be added to address the structural deficiency in that area. As such, these conventional radar altimeters have significant installation requirements.
Still other conventional radar altimeters are mounted to an aircraft such that the structure extends significantly outward (e.g., 0.75-3 inches) from the exterior of the aircraft. Such a structure can affect the aerodynamics of the aircraft and is also more directly exposed to weather.